Communications device including a filter for notching wideband receive signals and associated methods

ABSTRACT

A communications device includes pre-processing circuitry for processing a received wideband complex signal including an undesired narrowband interference component therein, and for determining a frequency of the undesired narrowband interference component. A filter is downstream from the pre-processing circuitry and operable to generate a received wideband complex signal with at least one frequency notch therein to suppress the undesired narrowband interference component. The filter includes a finite impulse response (FIR) filter with L taps to generate N output values, with L&gt;N. A Fast Fourier Transform (FFT) block is downstream from the FIR filter and has a length N so that filter transition regions occur between frequency bins of the FFT block. A notching block is downstream from the FFT block to generate the frequency notch. An Inverse Fast Fourier Transform (IFFT) block is downstream from the notching block and has the length N.

FIELD OF THE INVENTION

The present invention relates to the field of communications devices,and more particularly, to a communications device including a filter fornotching wideband receive signals to suppress undesired interferencecomponents therein.

BACKGROUND OF THE INVENTION

Wideband communications devices typically operate with a widebandcomplex signal, and can interfere with narrowband communicationsdevices. Interference with narrowband communications devices may cause aloss of critical communication links. For wideband and narrowbandcommunication devices to coexist, the wideband complex signal beingtransmitted may be filtered to generate notches overlapping theoperating frequencies of the narrowband communication devices.

An example wideband communications device is a satellite communicationssystem known as MOOS (Mobile User Objective System). The MOOS satellitecommunications system operates in the UHF band. The MUOS system isdesigned to reduce the impact on nearby narrowband communicationsdevices by using an adaptive notch-on-transmit filter.

There are several approaches for providing an adaptive notch-on-transmitfilter for notching the MUOS waveform. Each approach uses a differentfilter bank structure that affects processor loading and memoryrequirements. For example, a paper titled “Filter banks For AdaptiveTransmit Filtering” by Chad Spooner discloses notch-on-transmitalgorithms that focus on a DFT filter bank and on a modified DFT (mDFT)filter bank.

The DFT filter bank is applied to the waveform in a sliding blockmanner. In the mDFT, the input signal is simultaneously applied to eachof the sub-band branches of the filter bank. Analysis filters areimmediately applied to the input data. Each analysis filter is afrequency-shifted version of a real-valued low pass filter. The outputsof the analysis filters are decimated, and the real and imagery parts ofthe result are alternately taken over time. The parallel operationsresult in real and imagery components for each input block of complexnumbers, just as in the DFT filter bank. The difference between the mDFTand the DFT filter banks is that the exact sub-band filteringcharacteristics are under a designer's direct control in the mDFT filterbank. Even in view of the DFT filter bank and the modified DFT (mDFT)filter bank, processor loading and memory requirements are notsignificantly reduced.

A paper titled “MUOS Spectrum Notching Effect On Handheld TerminalUplink Performance” by Kumm et al. discusses the effect of thepeak-to-average power ratio (PAPR) of the spectrally adaptive waveformin terms of performance. The PAPR increases with an increasing notchingbandwidth in the spectrally adaptive waveform as compared to anun-notched waveform. With spectrally adaptive notching, the dynamicrange of the communications device transmitting the waveform is“squeezed” by the need to preserve PAPR while meeting a maximum powerlimit. The paper concludes that there is no straightforward way toreduce PAPR into the power amplifier of the communications devicetransmitting the waveform so as to boost output power of the requirednotch depth and out-of-band requirements. In addition, the paper failsto address reducing processor loading and memory requirements whengenerating the notching in the spectrally adaptive waveform.

In addition, wideband communications devices receiving a widebandcomplex signal are also susceptible to narrowband interference.Narrowband interference may result from the presence of background UHFinterference, legacy interferes and multiple access interference.Narrowband interference affects the signal-to-noise ratio (SNR) of thereceived wideband complex signal. A paper titled “MUOS U2B InterferenceMitigation Analysis” by Bahr et al. discloses how spectral whiteningapplied at the receiver reduces interference by reducing interferencepower at the demodulator. In the receiver, the received wideband complexsignal including an undesired narrowband interference component thereinis applied to a filter comprising a mDFT filter bank. The mDFT filterbank implements the spectral whitening while also notching the undesirednarrowband interference component. While the filter comprising the mDFTfilter bank is effective at suppressing the undesired narrowbandinterference component, the use of a mDFT filter still places a demandon processor loading requirements.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to adaptively notch a received wideband signal tosuppress narrowband interference while reducing processor loadingrequirements.

This and other objects, features, and advantages in accordance with thepresent invention are provided by a communications device comprisingpre-processing circuitry for processing a received wideband complexsignal including an undesired narrowband interference component therein,and for determining a frequency of the undesired narrowband interferencecomponent. A filter may be downstream from the pre-processing circuitryto generate a received wideband complex signal with at least onefrequency notch therein to suppress the undesired narrowbandinterference component.

The filter may comprise a finite impulse response (FIR) filtercomprising L taps to generate N output values, with L>N, and a FastFourier Transform (FFT) block downstream from the FIR filter and havinga length N so that filter transition regions occur between frequencybins of the FFT block. A notching block may be downstream from the FFTblock to generate at least one frequency notch. An Inverse Fast FourierTransform (IFFT) block may be downstream from the notching block andhaving the length N. A demodulator is downstream from the filter.

The pre-processing circuitry may comprise a down-converter forconverting the received wideband complex signal including the undesirednarrowband interference component therein to a baseband signal, a signalanalyzer coupled to the down-converter for analyzing the baseband signalfor determining the frequency of the undesired narrowband interferencecomponent.

The notching block may generate the at least one frequency notch for atleast partially removing the undesired narrowband interference componentin the received wideband complex signal based on an input from thepre-processing circuitry. The received wideband complex signal has aprofile that is different than a profile of the undesired narrowbandinterference component, and the notching block may generate the at leastone frequency notch so that a profile of the undesired narrowbandinterference component matches the profile of the received widebandcomplex signal.

The size of the FFT block is advantageously matched to the outputs ofthe FIR filter, and the FIR filter functions as a reference filter. Thereference filter is effectively being applied to each frequency bin inthe frequency domain as a window. This window is being applied acrossthe FFT history to implement a larger effective filter using the factthat the signal is also being decimated. Since the size of the FFT blockis matched to the outputs of the FIR filter, this advantageously reducesprocessor loading requirements.

The filter may operate with a block length of M, with M=N/2, and whereinthe N output values are filtered every M samples in time. The filter mayfurther comprise an interpolate block downstream from the IFFT. The FIRfilter may have an impulse response with zeros at N spaced values. Thefilter may have a filter gain halfway between frequency bins of the FFTblock with a magnitude of at least 0.5, and is anti-symmetric so that acomposite filter bank is spectrally flat.

The filter adaptively changes the at least one frequency notch. Thefilter may operate within a range of 300 to 3,000 MHz, for example. Thefilter may generate the output wideband complex signal having abandwidth within a range of about 4 to 5 MHz, for example.

Another aspect of the present invention is directed to a widebandcommunications device comprising a down-converter for converting areceived wideband complex signal including an undesired narrowbandinterference component therein to a baseband signal, a signal analyzerdownstream from the down-converter for analyzing the baseband signal fordetermining a frequency of the undesired narrowband interferencecomponent, and a filter downstream from the signal analyzer to generatea received wideband complex signal with at least one frequency notchtherein to suppress the undesired narrowband interference component.

The filter may comprise a finite impulse response (FIR) filtercomprising L taps to generate N output values, with L>N, and a FastFourier Transform (FFT) block downstream from the FIR filter and havinga length N so that filter transition regions occur between frequencybins of the FFT block. A notching block may be downstream from the FFTblock to generate at least one frequency notch, and an Inverse FastFourier Transform (IFFT) block may be downstream from the notching blockand having a length N. A demodulator may be downstream from the filter.

Yet another aspect of the present invention is directed to a method forgenerating a received wideband complex signal with at least onefrequency notch therein. The method comprises processing a receivedwideband complex signal including an undesired narrowband interferencecomponent therein, determining a frequency of the undesired narrowbandinterference component, and using a filter for filtering the widebandcomplex signal including the undesired narrowband interference componentto generate a received wideband complex signal with at least onefrequency notch therein to suppress the undesired narrowbandinterference component.

The filtering may comprise filtering the wideband complex signalincluding the undesired narrowband interference component therein usinga finite impulse response (FIR) filter comprising L taps to generate Noutput values, with L>N, and generating a Fourier transform of thewideband complex signal using a Fast Fourier Transform (FFT) blockdownstream from the FIR filter. The FFT block may have a length N sothat filter transition regions occur between frequency bins of the FFTblock.

The filtering may further comprise generating the at least one frequencynotch in the Fourier transform using a notching block downstream fromthe FFT block, and generating an inverse Fourier transform of theFourier transform with the at least one frequency notch therein using anInverse Fast Fourier Transform (IFFT) block downstream from the notchingblock, the IFFT having a length N. The received wideband complex signalwith at least one frequency notch therein may then be demodulated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a wideband communications device operatingas part of a satellite communications system in accordance with thepresent invention.

FIG. 2 is a graph illustrating a sampled signal spectrum received by thewideband communications device when in a scan mode in accordance withthe present invention.

FIG. 3 is a graph illustrating an output wideband complex signal with anotch generated therein based on the sampled signal spectrum shown inFIG. 2.

FIG. 4 is a flowchart for determining location of frequency notches tobe generated in an output wideband complex signal that is to betransmitted by a wideband communications device in accordance with thepresent invention.

FIG. 5 is a flowchart for generating an output wideband complex signalwith at least one frequency notch therein in accordance with the presentinvention.

FIG. 6 is a block diagram of a FIR filter in accordance with the presentinvention.

FIG. 7 is a block diagram illustrating a basic filter structure in whichthe filters are anti-symmetric in a transition region to provide a flatcomposite response in accordance with the present invention.

FIG. 8 is a block diagram of a wideband communications device receivinga wideband complex signal within a satellite communications system inthe presence of a narrowband device providing an undesired narrowbandinterference signal in accordance with the present invention.

FIG. 9 is a graph illustrating a received signal spectrum of thewideband complex signal received by the wideband communications deviceshown in FIG. 8 along with the undesired narrowband interference signaltherein.

FIG. 10 is a graph illustrating the received wideband complex signalwith a notch generated therein based on the received signal spectrumillustrated in FIG. 9 to suppress the undesired signal component.

FIG. 11 is a flowchart for determining location of frequency notches tobe generated in a received wideband complex signal in accordance withthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Referring initially to FIG. 1, a wideband communications device 20(1)will now be discussed. The illustrated wideband communications device20(1) communicates with another wideband communications device 20(2) viaa satellite 30. The wideband communications device 20(1), 20(2) may forma network of or be part of a network of wideband communications devices.The transmit link from the wideband communications devices 20(1) to thesatellite 30 is known as the uplink 22, whereas the receive link by theother wideband communications devices 20(2) from the satellite 30 isknown as the downlink 23. The frequency of the output wideband complexsignal for the uplink 22 is different than the frequency of the outputwideband complex signal for the downlink 23.

As will be discussed in greater detail below, each widebandcommunications device 20(1), 20(2) includes a filter 40 for notching theoutput wideband complex signal being transmitted to allow signalcoexistence with narrowband communications devices 50(1) and 50(2)operating in the general area. Each of the narrowband communicationsdevices 50(1), 50(2) includes an antenna 52, and communicationstherebetween is over link 54. The narrowband communications devices50(1), 50(2) have an operating bandwidth that is typically less than 100kHz, for example. In contrast, a bandwidth of the output widebandcomplex signal is typically in the MHz range, i.e., greater than 1 MHz.

The illustrated network of wideband communications devices 20(1), 20(2)is a satellite communications system. The satellite communicationssystem may be a MUOS (Mobile User Objective System) satellitecommunications system, for example. In a MUOS satellite communicationssystem, the bandwidth of the output wideband complex signal is about 5MHz. When filter roll-off is taken into consideration, the bandwidth isabout 4.7 MHz. The uplink transmits on frequencies near 310 MHz and thedown link receives on frequencies near 370 MHz for the MUOS satellitecommunications system.

The operating frequency of the filter 40 may be configured to cover theUHF band, i.e., within a range of 300 to 3,000 MHz. Nonetheless, thefilter 40 may be designed to operate over different frequency ranges asreadily appreciated by those skilled in the art. In addition, thebandwidth of the output wideband complex signal is not limited to 5 MHz.The bandwidth may vary within a range of 1 to 5 MHz, for example.

Interference of the transmitted output wideband complex signal from thewideband communications device 20(1) with the narrowband communicationsdevices 50(1), 50(2) may cause a loss of critical communications overlink 54. For the wideband and narrowband communications devices 20(1),20(2) and 50(1), 50(2) to co-exist, the output wideband complex signalis filtered before being transmitted to generate frequency notchesoverlapping the operating frequencies of the narrowband communicationdevices.

The output wideband complex signal may be a direct-sequence spreadspectrum signal, for example. Due to the large spreading gain of such asignal, a significant amount of the signal can be frequency notchedbefore serious communication degradation results. For example, up to 25%of the signal may be notched.

In the illustrated wideband communications device 20(1), the filter 40is between a modulator 38 and a transmitter 70. The filter 40 includes anotching block 46 for generating the output wideband complex signal withat least one frequency notch therein. The at least one frequency notchcorresponds to the operating frequency of the narrowband communicationsdevices 50(1), 50(2). Additional frequency notches may be formed in theoutput wideband complex signal by the notching block 46 based on theknown operating frequencies of other narrowband communications devicesoperating in the area, as readily appreciated by those skilled in theart.

The filter 40 comprises an input buffer 41 for buffering the modulatedsignal provided by the modulator 38, and a finite impulse response (FIR)filter 42 is downstream from the input buffer. The FIR filter 42comprises L taps to generate N output values, with L>N. A Fast FourierTransform (FFT) block 44 is downstream from the FIR filter 42 and has alength N so that filter transition regions occur between frequency binsof the FFT block.

The size of the FFT block 44 is advantageously matched to the outputs ofthe FIR filter 42, and the FIR filter 42 functions as a referencefilter. The reference filter is effectively being applied to eachfrequency bin in the frequency domain as a window. This window is beingapplied across the FFT history to implement a larger effective filterusing the fact that the signal is also being decimated. Because of“zeros” in the window function, a small number of multiples are requiredto create the output wideband complex signal. Consequently, processorloading and memory requirements may be advantageously reduced whenadaptively notching a wideband transmit signal to allow signalcoexistence with narrowband communication devices.

Normally, the FFT block 44 functions as a reference filter, as discussedin the background section. As a result of the FIR filter 42 performing,in effect, sub-band filtering, the FFT block 44 has a gain of 0 onefrequency bin away from a center frequency bin to reduce FFT leakage(e.g., splatter/spillover) between frequency bins in the FFT block 44.

The notching block 46 is downstream from the FFT block 44 to generatethe frequency notches in the output wideband complex signal.Determination of the frequency notches will be discussed in greaterdetail when reference is made to the flowchart 100 in FIG. 2.

An Inverse Fast Fourier Transform (IFFT) block 47 is downstream from thenotching block 46 and also has a length N. An output buffer 48 isdownstream from the IFFT block 47 for buffering the outputs therefrom.An interpolator 49 is downstream from the output buffer 48 forinterpolating values between the frequency bins. As part of theinterpolating, an overlay and add is performed to provide a high enoughsample rate to reconstruct the output of each of the N frequency bins.The sampling needs to be twice as fast as the bandwidth, as stated bythe Nyquest theorem. The transmitter 70 transmits the output widebandcomplex signal with the frequency notches therein via antenna 80.

A switch 72 is coupled between the transmitter 70 and the antenna 80.The switch 72 is also coupled to a receiver 74. A controller 76 iscoupled to the switch 72 for selectively connecting the antenna 80 tothe receiver 74, or connecting the antenna 80 to the transmitter 70. Thereceiver 74 is selected when the wideband communications device 20(1) isto operate in a scan mode, and the transmitter 70 is selected when thewideband communications device 20(1) is to operate in a transmit mode.

In the scan mode, the receiver 74 listens for narrowband communicationsdevices that are operating in the area. This includes the illustratednarrowband communications devices 50(1), 50(2) as well as other devicesoperating in the general area. For illustration purposes, FIG. 2illustrates a sampled signal spectrum 90 received by the widebandcommunications device 20(1) when in the scan mode. The sampled signalspectrum 90 includes a spike 92 corresponding to an operating frequencyof the narrowband communications devices 50(1), 50(2).

Since the wideband communications devices 20(1), 20(2) are networkedtogether, they both switch to the scan mode so that neither one istransmitting. The scan mode may also be referred to as a learning mode.The sampled signal spectrum 90 is provided by the receiver 74 to asignal analyzer 77. The signal analyzer 77 analyzes the sampled signalspectrum 90 for determining the location of where the frequency notchesare to be generated in the output wideband complex signal. Once thelocations of the frequency notches are determined, this information ispassed to the filter 40, and in particular, to the notching block 46.

In response to the sampled signal spectrum 90, the filter 40 provides anoutput wideband complex signal 94 with a frequency notch 96 generatedtherein as best illustrated in FIG. 3. The frequency notch 96 allows forthe spike 92 in the sampled signal spectrum 90. As noted above, thespike 92 corresponds to the operating frequency of the narrowbandcommunications devices 50(1), 50(2). Consequently, the frequency notch96 in the output wideband complex signal 94 allows the narrowbandcommunications devices 50(1), 50(2) to communicate while the widebandcommunications device 20(1) communicates with the other widebandcommunications device 20(2) via satellite 30.

Referring now to the flowchart 100 in FIG. 4, determining location ofthe frequency notches in the output wideband complex signal will bediscussed. From the start (Block 102), the receiver 74 is set to adesired sample frequency at Block 104, and the receiver digitallysamples a received signal at this frequency at Block 106. The sampledreceived signal is provided to the signal analyzer 77 that is coupled tothe receiver 74, and an EFT is performed by the signal analyzer on thesampled received signal at Block 108.

The signal analyzer 77 computes a magnitude of each frequency bin in theFFT at Block 110. The magnitudes may be computed directly, or may becomputed from approximation. A statistical value of the computedmagnitudes is computed at Block 112. The statistical value may be anaverage value or a median value, for example.

Before a determination can be made as to which frequency bins in the FFTare to be weighed with either a 1 or a 0, a weighting threshold iscomputed in Block 114. The weighting threshold, for example, may beequal to the computed statistical value multiplied by a profile of areference waveform that is also multiplied by a reference number. Theprofile of the reference waveform may be based on the sampled signalreceived in Block 106, which has a determined shape and width infrequency. The reference number may be 1.25 for example, whichcorresponds to 2 dB. Alternatively, the reference number may be a valueother than 1.25.

The magnitude of each frequency bin is compared to the weightingthreshold in Block 116. Each frequency bin is weighted with a 1 or a 0at Block 118 based on the comparing. If the magnitude of a frequency binis greater than the weighting threshold, then the frequency bin will beweighted with a 0. Alternatively, if the magnitude of a frequency bin isless than the weighting threshold, then the frequency bin will beweighted with a 1.

The number of frequency bins being zeroed is limited to a count limit atBlock 120. The count limit may correspond to a percentage of thefrequency bins, such as 25 percent, for example. The number of frequencybins being zeroed needs to be limited to allow sufficient room for theinformation to be transmitted in the output wideband complex signal andstill be recoverable. Selection of the frequency bins being limited maybe based on the ones having the largest magnitude.

Additional frequency bins may be zeroed at Block 122 based onfrequencies stored in a memory 78 within the signal analyzer 77. Thestored frequencies correspond to known narrowband communications devicesoperating in the area that may be inactive when the receiver 74 is inthe scan mode. Again, the number of frequency bins being zeroed may belimited to the having the largest magnitude. The zero window to beapplied in the notching block 46 is provided at Block 124. The flowchart100 ends at Block 126.

Referring now to the flowchart 200 in FIG. 5, generating an outputwideband complex signal with at least one frequency notch therein willbe discussed. From the start (Block 202), a complex input signal isprocessed as an M length block at Block 204. M is defined as being equalto N/2, where N=refers to the size of the N point complex FFT. Theprocess runs every M samples. The complex input signal is stored in ainput buffer 41 at Block 206. The filter has a length L, and the arraysare complex values. A new input frame of M complex input samples is alsostarted in the input buffer 41 at Block 208.

The array taps for the FIR filter 42 are defined at Block 210. The tapsmay be re-ordered for fast access. This allows straightforward pointerarithmetic that is typical of DSP and FPGA technology. The N pointcomplex FFT for the different tap arrays is computed by the FFT block 44at Block 212. The FIR filter 42 may have order taps h0 . . . hn-1, forexample, where increasing the order taps represent delayed scalar tapsof the desired filter response.

As illustrated in FIG. 6, the ordered filter of length L allows the tapsto be placed modulo N, where N is the length of the complex FFT size.The impulse filter length L is modulo in size N and represents thedesired low pass filter response needed to provide a flat compositefilter bank response. For illustration purposes, assume N=4 and L=16.The filter h0 . . . hn-1 would be reordered as follows: taps [0, . . .15]=[(h0, h4, h8, h12), (h1, h5, h9, h13), (h2, h6, h10, h14), (h3, h7,h11, h15)]. The ordered taps in each pair of parenthesis corresponds toa respective sub-filter. Because the input signal is being decimated,only parts of the signal are being examined, as readily appreciated bythose skilled in the art. The resulting values from the sub-filters aremultiplied and accumulated together in time to provide an input of theFFT block 44.

A basic filter structure for the FIR filter 42 is illustrated in FIG. 7.The FIR filter 42 may have a filter gain halfway between frequency bins51 of the FFT block 44 with a magnitude of at least 0.5, and isanti-symmetric so that a composite filter bank is spectrally flat. TheFFT block 44 may have a gain of 0 one frequency bin away from a centerfrequency bin to reduce FFT leakage between frequency bins. Reducingleakage between frequency bins advantageously reduces splatter andspillover, which improves performance of the filter.

The FFT bin zeroing rule is applied in the notching block 46 at Block214. The FFT bin zeroing rule is based on the flowchart 100 provided inFIG. 4. If frequency bins are zeroed, the output spectrum is a highquality reproduction of the input with a notch inserted at the desiredfrequency bin removed. A depth of the notch can be set at the desiredlevel when the zeroing window is not set to a zero value (value greaterthan zero, but less than 1.0). Alternatively, if the FIR filter 42 isdesigned properly and no frequency bins are zeroed, the output is a highquality reproduction of the input signal.

The N point complex IFFT is computed by the IFFT block 47 at Block 216.An output vector from the IFFT block 47 is provided to an output buffer48 at Block 218. The wideband output complex signal is reconstructed inthe interpolator 49 at Block 220. The interpolator 49 operates based ona SINC filter, which corresponds to (sin x)/x. The reconstructedwideband output complex signal with a notch generated therein is outputat Block 222. The loop variables for the next iteration are updated atBlock 224. The method ends at Block 226.

Another aspect is directed to a method for generating an output widebandcomplex signal 94 with at least one frequency notch 96 therein using acommunications device 20(1) comprising a modulator 38 and a filter 40downstream therefrom. The method may comprise filtering a widebandcomplex signal using a finite impulse response (FIR) filter 42comprising L taps to generate N output values, with L>N, and generatinga Fourier transform of the wideband complex signal using a Fast FourierTransform (FFT) block 44 downstream from the FIR filter 42. The FFTblock 44 may have a length N so that filter transition regions occurbetween frequency bins of the FFT block 44. The method may furthercomprise generating the at least one frequency notch 96 in the Fouriertransform using a notching block 46 downstream from the FFT block 44,and generating an inverse Fourier transform of the Fourier transformwith the at least one frequency notch 96 therein using an Inverse FastFourier Transform (IFFT) block 47 downstream from the notching block 46,with the IFFT having a length N.

Another aspect of the above-discussed filter is to apply a similarconcept to address narrowband interference excision. Narrowbandinterference may result from the presence of background UHFinterference, legacy interferes and multiple access interference.Narrowband interference affects the signal-to-noise ratio (SNR) of thereceived wideband complex signal. The filter may be configured toeliminate the region corresponding to the narrowband interference in thereceived wideband complex signal, or may be configured to make thereceived wideband complex signal look flat over the bandwidth.

Referring now to FIG. 8, a block diagram of the wideband communicationsdevice 20(2) receiving a downlink signal 23 (i.e., a wideband complexsignal) within the satellite communications system in the presence of anarrowband device 55 will now be discussed. The narrowband device 55includes an antenna 56 for transmitting an undesired narrowbandinterference signal 57. The narrowband device 55 may be a narrowbandcommunications device, similar to the narrowband communication devices50(1), 50(2) illustrated in FIG. 1. Alternatively, the narrowband device55 may be a jammer, for example.

In the illustrated wideband communications device 20(2), the filter 40is now between a demodulator 380 and pre-processing circuitry 280. Thefilter 40 includes a notching block 46 for generating the receivedwideband complex signal with at least one frequency notch therein. Theat least one frequency notch corresponds to the operating frequency ofthe narrowband device 55 providing the undesired narrowband interferencecomponent.

The pre-processing circuitry 280 includes a down-converter 282 forconverting the received wideband complex signal including the undesirednarrowband interference component therein to a baseband signal. A signalanalyzer 284 is coupled to the down-converter 282 for analyzing thebaseband signal for determining the frequency of the undesirednarrowband interference component.

The pre-processing circuitry 280 monitors the received wideband complexsignals in real time for determining if undesired interferencecomponents are, included therein. For illustration purposes, FIG. 9illustrates a received signal spectrum 500 received by the widebandcommunications device 20(2). The received signal spectrum 500 includes aspike (i.e., undesired interference component) 502 corresponding to anoperating frequency of the narrowband device 55.

As will be discussed in greater detail below, the purpose of thepre-processing circuitry 280 is to remove any narrowband interferencecomponents so that the received wideband complex signal is relativelyflat, i.e., it looks like noise. For illustration purposes, FIG. 10illustrates the received wideband complex signal 510 after filtering bythe filter 40 with a notch 512 generated therein based on the receivedsignal spectrum illustrated in FIG. 9 to suppress the undesired signalcomponent.

As noted above, the notching block 46 generates the at least onefrequency notch for eliminating the region corresponding to thenarrowband interference, or alternatively, for making the receivedwideband complex signal look flat over the bandwidth. In the laterapproach, the filter 40 partially removes the undesired narrowbandinterference component in the received wideband complex signal. Thereceived wideband complex signal has a profile that is different than aprofile of the undesired narrowband interference component, and thenotching block 46 generates the at least one frequency notch so that aprofile of the undesired narrowband interference component matches theprofile of the received wideband complex signal. In other words, thereceived signal should look like white noise over the bandwidth.

Referring now to the flowchart 300 in FIG. 11, determining location ofthe frequency notches in the received wideband complex signal will bediscussed. From the start (Block 302), the down-converter 282down-converts the received signal spectrum (i.e., wideband complexsignal 500 including the undesired narrowband interference component 502therein) to a baseband (I & Q) signal at Block 304. The signal analyzer284 performs an FFT on the baseband signal and computes a magnitude ofeach frequency bin in the FFT at Block 306. The magnitudes may becomputed directly, or may be computed from approximation. A statisticalvalue of the computed magnitudes is computed at Block 308. Thestatistical value may be an average value or a median value, forexample.

Before a determination can be made as to which frequency bins in the FFTare to be weighed with either a 1 or a 0, a weighting threshold iscomputed in Block 310. The weighting threshold, for example, may beequal to the computed statistical value multiplied by a profile of areference waveform that is also multiplied by a reference number. Theprofile of the reference waveform may be based on the received widebandcomplex signal, which has a determined shape and width in frequency. Thereference number may be 1.25 for example, which corresponds to 2 dB.Alternatively, the reference number may be a value other than 1.25.

The magnitude of each frequency bin is compared to the weightingthreshold in Block 312. Each frequency bin is weighted with a 1 or a 0at Block 314 based on the comparing. If the magnitude of a frequency binis greater than the weighting threshold, then the frequency bin will beweighted with a 0. Alternatively, if the magnitude of a frequency bin isless than the weighting threshold, then the frequency bin will beweighted with a 1.

The number of frequency bins being zeroed is limited to a count limit atBlock 316. The count limit may correspond to a percentage of thefrequency bins, such as 25 percent, for example. The number of frequencybins being zeroed needs to be limited to allow sufficient room for theinformation to be recovered in the received wideband complex signal.Selection of the frequency bins being limited may be based on the oneshaving the largest magnitude.

The zero window to be applied in the notching block 46 is provided atBlock 124. However, since the pre-processing circuitry 280 is to makethe received wideband complex signal look like nose, it is not necessaryto completely remove the narrowband interference component since thisportion of the signal spectrum also includes a portion of theinformation signal to be recovered. Consequently, the zero-window ismodified at Block 318 so that the output of the FFT block 44 in thefilter 40 appears as a white signal. For example, if the zeroing windowis a 0 for BINi, then the complex signal for BINi is scaled to have amagnitude of a median value but retain the phase of the complex value,as follows:

Re[complex bin(i)]=Re[complex bin(i)]*median value/mag(complexbin(i))

Im[complex bin(i)]=Im[complex bin(i)]*median value/mag(complexbin(i))

Similar to the filter 40 on the transmit side, regions of the signalthat exceed the zeroing threshold can be used to suppress narrowbandinterference components on the receive side. Instead of completelyeliminating the portion of the received signal with the narrowbandinterference component therein, the filter 40 filters the receivedsignal so that the narrowband interference component is suppressed sothat a profile thereof matches a profile of the other portions of thereceived signal. The flowchart 300 ends at Block 320.

After determining the modified zero window to be applied to the notchingblock 46, reference is now directed to the flowchart 200 illustrated inFIG. 5. The steps for generating a received wideband complex signal 510with at least one frequency notch 512 therein is similar to the stepsperformed on the transmit side of the wideband communications device20(1). These steps will not be discussed due to the similaritytherebetween, as readily appreciated by those skilled in the art.

The filter 40 comprises an input buffer 41 for buffering the receivedwideband complex signal with the interference component therein asprovided by the processing circuitry 280. The finite impulse response(FIR) filter 42 is downstream from the input buffer 41. The FIR filter42 comprises L taps to generate N output values, with L>N. A FastFourier Transform (FFT) block 44 is downstream from the FIR filter 42and has a length N so that filter transition regions occur betweenfrequency bins of the FFT block.

The size of the FFT block 44 is advantageously matched to the outputs ofthe FIR filter 42, and the FIR filter 42 functions as a referencefilter. The reference filter is effectively being applied to eachfrequency bin in the frequency domain as a window. This window is beingapplied across the FFT history to implement a larger effective filterusing the fact that the signal is also being decimated. Since the sizeof the FFT block 44 is matched to the outputs of the FIR filter 42, thisadvantageously reduces processor loading requirements. The notchingblock 46 is downstream from the FFT block 44 to generate the frequencynotches in the output wideband complex signal.

An Inverse Fast Fourier Transform (IFFT) block 47 is downstream from thenotching block 46 and also has a length N. An output buffer 48 isdownstream from the IFFT block 47 for buffering the outputs therefrom.An interpolator 48 is downstream from the output buffer 48 forinterpolating values between the frequency bins. As part of theinterpolating, an overlay and add is performed to provide a high enoughsample rate to reconstruct the output of each of the N frequency bins.The sampling needs to be twice as fast as the bandwidth, as stated bythe Nyquest theorem. The demodulator 380 demodulates the receivedwideband complex signal with the at least one frequency notch therein asoutput by the filter 40.

Another aspect is directed to a method for generating a receivedwideband complex signal 510 with at least one frequency notch 512therein. The method comprises processing a received wideband complexsignal 500 including an undesired narrowband interference component 502therein, determining a frequency of the undesired narrowbandinterference component, and using a filter 40 for filtering the widebandcomplex signal including the undesired narrowband interference componentto generate a received wideband complex signal 510 with at least onefrequency notch 512 therein to suppress the undesired narrowbandinterference component 502.

The filtering may comprise filtering the wideband complex signalincluding the undesired narrowband interference component therein usinga finite impulse response (FIR) filter 42 comprising L taps to generateN output values, with L>N, and generating a Fourier transform of thewideband complex signal using a Fast Fourier Transform (FFT) block 44downstream from the FIR filter 42. The FFT block 44 may have a length Nso that filter transition regions occur between frequency bins of theFFT block.

The filtering may further comprise generating the at least one frequencynotch 512 in the Fourier transform using a notching block 46 downstreamfrom the FFT block 44, and generating an inverse Fourier transform ofthe Fourier transform with the at least one frequency notch 512 thereinusing an Inverse Fast Fourier Transform (IFFT) block 47 downstream fromthe notching block 46, with the IFFT having a length N. The receivedwideband complex signal with at least one frequency notch therein maythen be demodulated.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A communications device comprising: pre-processing circuitry operablefor processing a received wideband complex signal including an undesirednarrowband interference component therein, and for determining afrequency of the undesired narrowband interference component; a filterdownstream from said pre-processing circuitry and operable to generate areceived wideband complex signal with at least one frequency notchtherein to suppress the undesired narrowband interference component,said filter comprising a finite impulse response (FIR) filter comprisingL taps to generate N output values, with L>N, a Fast Fourier Transform(FFT) block downstream from said FIR filter and having a length N sothat filter transition regions occur between frequency bins of said FFTblock, a notching block downstream from said FFT block and operable togenerate at least one frequency notch, and an Inverse Fast FourierTransform (IFFT) block downstream from said notching block and havingthe length N; and a demodulator downstream from said filter.
 2. Thecommunications device according to claim 1 wherein said pre-processingcircuitry comprises: a down-converter for converting the receivedwideband complex signal including the undesired narrowband interferencecomponent therein to a baseband signal; and a signal analyzer coupled tosaid down-converter for analyzing the baseband signal for determiningthe frequency of the undesired narrowband interference component.
 3. Thecommunications device according to claim 1 wherein said notching blockgenerates the at least one frequency notch for at least partiallyremoving the undesired narrowband interference component in the receivedwideband complex signal based on an input from said pre-processingcircuitry.
 4. The communications device according to claim 1 wherein thereceived wideband complex signal has a profile that is different than aprofile of the undesired narrowband interference component, and whereinsaid notching block generates the at least one frequency notch so that aprofile of the undesired narrowband interference component matches theprofile of the received wideband complex signal.
 5. The communicationsdevice according to claim 1, wherein said filter operates with a blocklength of M, with M=N/2, and wherein the N output values are filteredevery M samples in time.
 6. The communications device according to claim1, wherein said filter further comprises an interpolate block downstreamfrom said IFFT block.
 7. The communications device according to claim 1,wherein said filter has a filter gain halfway between frequency bins ofsaid FFT block with a magnitude of at least 0.5, and is anti-symmetricso that a composite filter bank is spectrally flat.
 8. Thecommunications device according to claim 1, wherein said filteradaptively changes the at least one frequency notch.
 9. Thecommunications device according to claim 1, wherein said filter operateswithin a range of 300 to 3,000 MHz.
 10. A wideband communications devicecomprising: a down-converter operable for converting a received widebandcomplex signal including an undesired narrowband interference componenttherein to a baseband signal; a signal analyzer downstream from saiddown-converter and operable for analyzing the baseband signal fordetermining a frequency of the undesired narrowband interferencecomponent; a filter downstream from said signal analyzer and operable togenerate a received wideband complex signal with at least one frequencynotch therein to suppress the undesired narrowband interferencecomponent, said filter comprising a finite impulse response (FIR) filtercomprising L taps to generate N output values, with L>N, a Fast FourierTransform (FFT) block downstream from said FIR filter and having alength N so that filter transition regions occur between frequency binsof said FFT block, a notching block downstream from said FFT block togenerate at least one frequency notch, and an Inverse Fast FourierTransform (IFFT) block downstream from said notching block and havingthe length N; and a demodulator downstream from said filter.
 11. Thewideband communications device according to claim 10 wherein saidnotching block generates the at least one frequency notch for at leastpartially removing the undesired narrowband interference component inthe received wideband complex signal based on an input from said signalanalyzer.
 12. The wideband communications device according to claim 10wherein the received wideband complex signal has a profile that isdifferent than a profile of the undesired narrowband interferencecomponent, and wherein said notching block generates the at least onefrequency notch so that a profile of the undesired narrowbandinterference component matches the profile of the received widebandcomplex signal.
 13. The wideband communications device according toclaim 10, wherein said filter operates with a block length of M, withM=N/2, and wherein the N output values are filtered every M samples intime.
 14. The wideband communications device according to claim 10,wherein said filter further comprises an interpolate block downstreamfrom said IFFT block.
 15. The wideband communications device accordingto claim 10, wherein said filter adaptively changes the at least onefrequency notch.
 16. The wideband communications device according toclaim 10, wherein said filter operates within a range of 300 to 3,000MHz.
 17. A method for generating a received wideband complex signal withat least one frequency notch therein, the method comprising: processinga received wideband complex signal including an undesired narrowbandinterference component therein; determining a frequency of the undesirednarrowband interference component; using a filter for filtering thewideband complex signal including the undesired narrowband interferencecomponent to generate a received wideband complex signal with at leastone frequency notch therein to suppress the undesired narrowbandinterference component, the filtering comprising filtering the widebandcomplex signal including the undesired narrowband interference componenttherein using a finite impulse response (FIR) filter comprising L tapsto generate N output values, with L>N, generating a Fourier transform ofthe wideband complex signal using a Fast Fourier Transform (FFT) blockdownstream from the FIR filter, the FFT block having a length N so thatfilter transition regions occur between frequency bins of the FFT block,generating the at least one frequency notch in the Fourier transformusing a notching block downstream from the FFT block, and generating aninverse Fourier transform of the Fourier transform with the at least onefrequency notch therein using an Inverse Fast Fourier Transform (IFFT)block downstream from the notching block, the IFFT having the length N;and demodulating the received wideband complex signal with at least onefrequency notch therein.
 18. The method according to claim 17 whereinthe processing comprises down-converting the received wideband complexsignal including the undesired narrowband interference component thereinto a baseband signal; and wherein the determining comprises analyzingthe baseband signal for determining the frequency of the undesirednarrowband interference component.
 19. The method according to claim 17wherein the notching block generates the at least one frequency notchfor at least partially removing the undesired narrowband interferencecomponent in the received wideband complex signal based on thedetermined frequency of the undesired narrowband interference component.20. The method according to claim 17 wherein the received widebandcomplex signal has a profile that is different than a profile of theundesired narrowband interference component, and wherein the notchingblock generates the at least one frequency notch so that a profile ofthe undesired narrowband interference component matches the profile ofthe received wideband complex signal.
 21. The method according to claim17, wherein the filter operates with a block length of M, with M=N/2,and wherein the N output values are filtered every M samples in time.22. The method according to claim 17, wherein the filter furthercomprises an interpolate block downstream from the IFFT block forinterpolating outputs therefrom.
 23. The method according to claim 17,wherein the filter has a filter gain halfway between frequency bins ofthe FFT block with a magnitude of at least 0.5, and is anti-symmetric sothat a composite filter bank is spectrally flat.
 24. The methodaccording to claim 17, wherein the filter adaptively changes the atleast one frequency notch.
 25. The method according to claim 17, whereinthe filter operates within a range of 300 to 3,000 MHz.